While a Charpy test has been commonly used as a standard for evaluating the toughness of steel, recently, a crack tip opening displacement test (hereinafter, referred to as “CTOD testing”) has become increasingly used for evaluating, with higher accuracy, the fracture resistance of a thick steel plate used for constructing structures. In CTOD testing, a test specimen having a fatigue crack formed in a toughness-evaluation portion of the test specimen is subjected to a bending test at a low temperature and an opening displacement (i.e., amount of plastic deformation) at the crack tip which occurs immediately before fracture is measured in order to evaluate resistance to brittle fracture.
When a structure is constructed using a thick steel plate, multipass welding is employed. It is known that a heat affected zone formed by multipass welding (hereinafter, referred to as “multipass weld HAZ”) includes a zone having considerably low toughness (hereinafter, referred to as “inter critically reheated coarse grain heat affected zone (ICCGHAZ)”), which includes a coarse base microstructure and an island-like martensite (i.e., martensite-austenite constituent (MA)) microstructure mixed in the coarse base microstructure. The ICCGHAZ is formed by reheating a zone in which a coarse microstructure is formed in the vicinity of the weld line by the preceding weld pass (i.e., coarse grain heat affected zone (CGHAZ)) to the ferrite-austenite dual phase region in the weld pass for the following layer.
In general, CTOD testing of welded joints examines a steel plate over its entire thickness. Therefore, when the multipass weld HAZ is examined, an evaluation zone in which the fatigue crack is to be formed includes the ICCGHAZ microstructure. The CTOD property of welded joints measured by CTOD testing of welded joints is affected by the toughness of a zone that has become the most brittle among the evaluation zone even the area of such a zone is small. Consequently, not only the toughness of the CGHAZ microstructure but also the toughness of the ICCGHAZ microstructure affects the CTOD property of welded joints in the multipass weld HAZ. Thus, in order to enhance the CTOD property of welded joints in the multipass weld HAZ, an increase in the toughness of the ICCGHAZ microstructure is also required.
In order to increase the heat-affected-zone (also referred to as “HAZ”) toughness, a technique in which coarsening of the austenite grains in the CGHAZ is prevented from occurring by dispersing TiN in the form of fine particles and a technique in which the TiN particles are used as nuclei for ferrite transformation have been used.
In addition, a technique in which the growth of the austenite grains is limited by dispersing a REM-based oxysulfide, which is produced by addition of a REM; a technique in which the growth of the austenite grains is limited by dispersing a Ca-based oxysulfide, which is produced by addition of Ca; and a technique in which the ferrite-nucleation-capability of BN and dispersion of an oxide are used in combination have also been used.
For example, Patent Literature 1 and Patent Literature 2 propose a technique in which coarsening of the austenite microstructure in the HAZ is prevented from occurring by using REM and TiN particles. Patent Literature 3 proposes a technique in which CaS is used for increasing the HAZ toughness and a technique in which hot rolling is performed for increasing the toughness of the base metal.
There has also been proposed a technique (e.g., Patent Literature 4) in which, in order to address the reduction in the ICCGHAZ toughness, formation of MA is limited by reducing the C and Si contents and the strength of the base metal is increased by adding Cu. Patent Literature 5 proposes a technique in which the grain refinement of the HAZ microstructure is achieved by using BN particles as nuclei for ferrite transformation in the large-heat-input heat affected zone in order to increase the HAZ toughness.